In recent years there have been many attempts to use long cylindrical optical transmission mediums as a means to transmit images for facsimiles, copiers and image sensors, etc. For such optical transmission mediums, it is desirable to have a product that can transmit high resolution and high quality images with little color aberration, and it is desirable to be able to adopt a manufacturing method for mass production which has few product quality variations among the optical transmission mediums.
With this kind of optical transmission medium for image transmission, a GI type optical transmission medium is used in which the refractive index becomes continuously smaller going from the center to the outer periphery within the cross-section of the optical transmission medium and in which there is an angled gradient in the distribution of the refractive index. This kind of GI type optical transmission medium appears in Japan Patent Publication No. Sho 47-816, Japan Patent Publication No. Sho 47-28059, and in EP Laid-Open Patent Application No. 208159, etc.
The GI type optical transmission medium indicated in Japan Patent Publication No. Sho 47-816 uses glass as the material, and because it is made by an ion exchange method, it has low productivity, and it is difficult to produce products between varying lots that provide the same shape and the same performance. In particular, with long cylindrical lenses of the same length that have a fixed total conjugate length, it is difficult to make products that provide the same performance between varying lots. As a result, the difficulties arise that the lengths of the GI type optical transmission mediums which provide the same performance are not uniform, and that handling them becomes troublesome.
The GI type plastic optical transmission mediums that are indicated in Japan Patent Publication No. Sho 47-28059 are made by dipping into a particular solvent long cylindrical shaped or fiber shaped molded products consisting of a mixture of 2 or more transparent polymers that have different refractive indices and which have different solubility in relation to said solvent, thus changing the percentage of mixture of the aforementioned 2 or more polymers going from the surface of said molding to its center. Plastic GI type optical transmission mediums can more or less be made by this method, but products that are a mixture of 2 or more polymers having different refractive indices can easily produce turbulence in the distribution of the refractive index, and it is difficult to make a product with a shape in which the refractive index distribution follows the optimum distribution curve from the core toward the outer periphery. Moreover products are made in which the transparency of the optical transmission medium is lowered, and the light scattering can easily occur. Thus, the characteristics are not sufficient as a GI type optical transmission medium, and this does not promote applications and development.
In EP Laid-Open Patent Application No. 0208159, a method is indicated in which, after molding into a rod shape a uniform mixture of at least one kind of thermoplastic polymer (A) and of monomer (B) which when polymerized is compatible with polymer (A) and which becomes a polymer with a refractive index that differs from polymer (A), and after making a continuous concentration distribution of monomer (B) going from the surface of said molded product to the center by dispersing monomer (B) from the surface of the molded product, a GI type plastic optical transmission medium made by polymerizing the unpolymerized monomer in said molded product.
The refractive index distribution curve of a GI type optical transmission medium should ideally have the quadratic distribution curve which is expressed by the following formula (3), and said curve should be like that shown by A in FIG. 2. EQU N=N.sub.0 (1-ar.sup.2) (3)
In this connection, the inventors of present invention found that, when using an interfaco-interference microscope to measure GI type optical transmission mediums that were made by the above mentioned conventional methods, the refractive index distribution curve was like B in FIG. 2, and the range of 0.5 r.sub.0 .about.0.75 r.sub.o radially from the core (In the same figure, this is the range of c.about.d; likewise, e indicates the periphery.) has a refractive index distribution curve with a shape that is rather close to the optimum curve expressed in formula (3), but the refractive index distribution further inside or outside of that greatly diverges from the optimum curve.
When observing a grid pattern using optical transmission mediums which have a refractive index distribution curve which almost perfectly follows the optimum curve stipulated in Formula (3), the regular grid image shown in FIG. 3 (a) can be obtained, but when observing a grid image using optical transmission medium in which the refractive index distribution varies from the ideal refractive index distribution as shown in FIG. 2B described above, only the greatly distorted grid images shown in FIGS. 3 (b) and (c) can be obtained, and accurate image transmission cannot be performed. The resolution of this kind of optical transmission medium can be expressed by a modulation transfer factor (abbreviated MTF hereafter). The calculation of the MTF value both of optical transmission mediums made by conventional technology and of optical transmission mediums of the present invention is as follows: first, as shown in FIG. 4, light from light source (42) is adjusted by lens (43), light which passes through grid (45) with a grid constant 4 (line pair/mm) strikes the GI type optical transmission medium (41), and the grid image that passes through this optical transmission medium is read by CCD sensor (46); the maximum value i.sub.max and the minimum value i.sub.min of this measured light quantity are measured as shown in FIG. 5; and the MTF value is calculated by Formula 4 below. Here, as shown by the grid in FIG. 4, the grid constant is a value which indicates how many line pairs, with 1 line pair being 1 group combining a white line and a black line, can fit within 1 mm, and the 4 lines/mm of this line pair is expressed as `4 line pair/mm`. EQU MTF (%)={(i.sub.max -i.sub.min)/(i.sub.max +i.sub.min)}.times. 100(4)
A GI type plastic optical transmission medium which has a refractive index distribution which almost exactly conforms to the optimum curve stipulated in aforementioned Formula (3) has not yet been developed.
Prior to the present invention, the inventors conducted studies to obtain GI type plastic optical transmission mediums with a resolution high enough and a color aberration small enough to be that they could be used practically as optical transmission mediums for facsimiles and image sensors when monochromatic light source such as LED is used, and proposed in Japan Patent Application No. Hei 1-307636 a GI type plastic optical transmission medium characterized by: having a radius r.sub.o in the range of 0.5.+-.0.1 mm; having a refractive index distribution from the central axis portion to the peripheral surface in the range of at least 0.25 r.sub.0 .about.0.70 r.sub.o which provides a refractive index distribution close to the refractive index distribution curve stipulated in the aforementioned Formula (3); and having an MTF of 40% or more when measuring the maximum light quantity value i.sub.max and minimum light quantity value i.sub.min by passing a 4 line pair/mm grid image through said optical transmission medium and focussing the image on a CCD sensor, and then calculating the MTF by the aforementioned Formula (4).
In this invention, the refractive index distribution in the radial direction of the GI type optical transmission medium achieved is like that of [II] in FIG. 1; it has many portions that agree with curve [I] which follows the optimum curve of Formula (3); and when actually transmitting images by this optical transmission medium, the performance was remarkably improved compared to the characteristics of the image transmissions of plastic optical transmission mediums that were developed conventionally.
However, when taking r.sub.o to be the radius from the center of the optical transmission medium, the refractive index of the peripheral portion outer than 0.70 r.sub.o of the GI type optical transmission medium diverged greatly from the optimum curve of Formula (3), and the peripheral area of the images transmitted by this kind of optical transmission medium had distortions and weakness and could not be said to be satisfactory as an optical transmission medium for image transmission of high resolution images. In order to eliminate this kind of fault, we studied methods to blacken the outer periphery area from 0.70 r.sub.o, but when that effect was obtained, the problem arose that the image transmission effectiveness of the optical transmission medium as a whole was reduced by the darkening for that portion only, and this could not be considered adequate.